1. Field of the Invention
This invention relates to a method for producing a support for a printing plate and more particularly to a method for producing an aluminum support for a printing plate in which an undesirable increase in adhesion between the support and a photosensitive material thereon with the passage of time is minimized.
2. Description of the Prior Art
When an aluminum plate having an anodized surface is employed as a support for a printing plate, the adhesion between the support and the photosensitive material provided thereon is generally insufficient, thus the resulting printing plate necessarily does not perform well. Hence the anodized oxide layer is usually etched in order to improve the adhesion between the support and the photosensitive material. For example, phosphoric acid is used to etch an anodized oxide surface layer as is set forth in German patent application OLS No. 2,251,180, Belgian Pat. No. 797,180, etc. Still other etching methods are known whereby various acids other than phosphoric acid and alkalis are used.
It is also known that certain cathodic electrolytic treatments can be used to improve adhesion to an aluminum plate with an anodized surface. However, when an aluminum plate having such an anodized surface which has been further subjected to an etching treatment is used in the production of a photosensitive printing plate, the adhesion between the support and the photosensitive material still gradually increases to such an extent that the developability is deteriorated and that the photosensitive material at the background areas (nonimage areas) tends to remain after development, thus causing background stain in the final printed materials. Particularly when such a printing plate has been stored before use under high temperature and high humidity conditions for a prolonged period, development is not possible at all in some cases.
To overcome these disadvantages in printing plates, the plates are subjected to a so-called "hydrophilic treatment", which is well known in the art.
A hydrophilic treatment includes immersion in various aqueous solutions containing one of sodium silicate, potassium fluorozirconate, phosphate glass, a heavy metal salt, a chromium compound, etc., or coating of a hydrophilic polymer material on the plate as a subbing layer.
Suitable hydrophilic polymer materials for such a purpose include poly(vinylbenzenesulfonic acid), poly(acrylic acid), carboxymethylcellulose, poly(acrylamide), poly(vinyl alcohol), gum arabic, poly(vinylpyrollidone), a poly(ethylene/maleic anhydride) copolymer, etc. Sub-coating can be carried out using an aqueous solution containing at least one of these materials.
The above-described increase in adhesion during storage of a printing plate cannot be completing suppressed even by treatment of the support in the manner described above and, ultimately, the plate begins to exhibit poor developability, giving printed materials with a stained, soiled background.
Further, treating solutions containing sodium silicate, potassium fluorozirconate, or heavy metal salts or chromium compounds have other drawbacks from the standpoint of the difficulty and complications in processing the waste solution for pollution prevention and also potential danger to the operator's health.